Will pre-lithiation become Tesla’s killer feature?

Musk not only wants to build electric cars, but also wants to become the king of power batteries. One of his capitals is the supercapacitor company MAXWELL that he acquired. MAXWELL has two technologies that attract attention. One is dry electrode technology, and the other is a pre-lithiation technology that has not received much attention.

Maxwell claims that dry electrode technology can increase battery energy density to 300Wh/kg, and is expected to reach 500Wh/kg in the future. To increase battery energy density to 500Wh/kg, it is likely that pre-lithiation technology will be used to assist. At Tesla's upcoming "Battery Day", some analysts believe that Tesla will announce pre-lithiation technology.

What is pre-lithiation technology? If Tesla is optimistic about it, are other companies doing it besides Maxwell? Can it be quickly applied?

What is pre-lithiation technology?

To understand pre-lithiation technology, we must first know why batteries need pre-lithiation.

Generally speaking, during the first charging process of lithium-ion power batteries, the organic electrolyte will be reduced and decomposed on the surface of the negative electrode such as graphite to form a solid electrolyte interface (SEI) film, which consumes lithium from the positive electrode, resulting in a low coulombic efficiency (ICE) of the first cycle, reducing the capacity of the lithium-ion power battery and thus affecting the energy density.

In simple terms, when a lithium-ion battery is charged for the first time, a large amount of lithium loss will occur, and this is irreversible. In order to ensure the capacity of the battery, some of the lost lithium needs to be replenished.

From the perspective of technical path, the current mainstream lithium replenishment solutions can be divided into two categories: one is negative electrode lithium replenishment, which is mainly inert metal lithium powder, metal lithium foil or lithium compounds; the other is positive electrode lithium replenishment, which is mainly some lithium-containing oxides.

Why is pre-lithium necessary?

Improve battery capacity and cycle life.

If a graphite negative electrode is used, companies believe that there is little point in replenishing lithium because the coulombic efficiency of the graphite negative electrode is acceptable.

For silicon anode, too much lithium is lost in the first cycle of charge and discharge. Pre-lithiation is to compensate for the lithium loss and extend the cycle life, thereby slowing down the attenuation.

Why do we use silicon negative electrode? This starts with improving energy density.

Generally speaking, in order to increase the energy density of battery cells, it is necessary to use positive and negative electrode materials with high specific capacity.

Specifically, the method to improve the energy density of the positive electrode is to use high-nickel positive electrode materials, such as NCM811, NCA and lithium-rich manganese-based materials, which are the current main directions; in terms of negative electrode materials, the porous and fluffy graphite is replaced with a silicon-based negative electrode with higher specific capacity, as well as a metallic lithium negative electrode.

Theoretical specific capacity of three negative electrode materials:

Graphite-based: 372mAh/g

Silicon-based: 3580mAh/g (room temperature)

Lithium metal: 3860mAh/g

Since the technology of metal lithium anode is too difficult and too far away, silicon-based anode is currently the most promising anode material.

However, the disadvantage of using pure silicon as the negative electrode is also very obvious, which is that it will cause a high battery expansion rate. The silicon negative electrode can expand by up to 360% during charge and discharge, while ordinary graphite is only 10%. This will cause the negative electrode to decay rapidly during the cycle. This is because some lithium ions cannot be deintercalated from the negative electrode back to the positive electrode, which results in lithium loss. To put it simply, the battery cell expands and contracts too many times, the structure collapses, and lithium cannot enter or exit.

What is the lithium loss of graphite negative electrode and silicon negative electrode in the first week of charging and discharging?

Research has found that existing graphite materials have an initial irreversible lithium loss of 5% to 10%, and for high-capacity negative electrode materials, the initial lithium loss is even higher; the irreversible capacity loss of silicon is as high as 15% to 35%.

It can be seen that the capacity of the negative electrode is improved by using pure silicon, but the cycle life is too short.

At present, a relatively mature technical solution is to use carbon materials with small volume effect and good cycle stability as carriers, and incorporate silicon materials with high specific capacity as the main active body to synthesize silicon-carbon composite materials.

Another problem arises. As people continue to pursue high-capacity batteries, the silicon content must continue to increase, and the shortcoming of short cycle times becomes increasingly unbearable.

In order to slow down the attenuation of battery capacity, pre-lithium technology comes in handy.

Although the law of battery degradation cannot be changed, the initial lithium loss can be compensated through lithium replenishment technology.

Mr. Deng, a pre-lithium technology expert at a power battery company, said that the silicon content in the currently commonly used graphite negative electrode materials is 3%-7%. Since the initial efficiency of the silicon negative electrode is only 50%, the initial efficiency of the silicon-carbon negative electrode will gradually decrease with the increase of the silicon content. When the silicon content reaches 10%, it is necessary to use pre-lithium technology to increase the battery capacity.

Source: Wanxiang Patent 201510029061.5 Pre-lithiation treatment effect diagram

Since the direction of pre-lithiation is very clear, companies at home and abroad are looking for the lowest-cost and safest pre-lithiation solution.

According to Mr. Lin, technical director of negative electrode materials at Gotion High-tech Engineering Research Institute, there are actually two ideas to solve the cycle life of silicon negative electrodes. One is to find a solution in the silicon negative electrode material itself. Some companies are currently trying, but it is too difficult. The second is pre-lithiation, which is the most direct and effective.

Currently in the laboratory research stage, electrochemical pre-lithiation and direct contact short-circuit method are simple and effective methods, which effectively mitigate the first irreversible loss of high-capacity carbon materials, alloy negative electrodes and conversion materials. They have the advantages of precise control of pre-lithiation amount and good stability, but have high requirements for the environment, such as oxygen-free, water-free and dry environment, which limits their large-scale application.

Using stable metallic lithium powder for pre-lithiation is currently the most effective and direct method for commercialization. However, it has very high requirements for the production environment and requires the development of closed slurry mixing equipment to avoid the safety hazards of burning electrode materials, conductive agents, etc. caused by high-speed stirring, which poses a great risk in the process.

In addition, high cost is also one of the difficulties in commercial application.

It can be seen that the process of battery development is a process of breaking the original balance and then creating a new balance. In order to improve a certain characteristic, we adopt the strengths of a new element and use other methods to patch up the shortcomings of this element. Of course, this will also bring new problems, and the cycle will repeat.

Research on Pre-lithiation Technology in China

Since it is a consensus in the pre-lithium industry, Chinese companies are naturally also exploring it.

At present, pre-lithiation has become an important research and development field for many negative electrode material manufacturers and battery companies. Mainstream battery companies and research institutes have reserved many patents in this field.

Source: soopat

For example, CATL has a number of patents, one of which is for replenishing lithium powder in the negative electrode of lithium-ion batteries: first, lithium powder is sprinkled in a closed space through a feeding device; after spraying, the baffle and DC power supply are opened, and the lithium powder is evenly sprayed on the surface of the negative electrode under the action of vibration and power plant; the amount of lithium replenishment is controlled by controlling the belt speed; and the lithium powder and the negative electrode are pressed together again by roller pressing.

Guoxuan High-tech pre-lithiates the negative electrode of lithium-ion batteries. The negative electrode, separator, and lithium sheet are placed in the electrolyte in sequence, and the lithium sheet does not touch the negative electrode. Secondly, an external power supply is used to charge the negative electrode, and the current is controlled at 0.05-2C and the charging time to achieve the purpose of lithium replenishment. Finally, the electrode is dried to obtain the pre-lithiated negative electrode.

Source: Guoxuan High-tech patent CN201910418237.4

Wanxiang A123's method is to first prepare a silicon-carbon negative electrode; secondly, in a glove box, use a two-step constant current pulse deposition method to electro-deposit metallic lithium in an electrolytic cell; thirdly, immerse it in DMC to wash away the surface lithium; finally, dry it to obtain a pre-lithiated electrode; and by controlling the current size and time, the pre-lithiated silicon-carbon negative electrode of different thicknesses can be deposited.

The above are just a few examples of patented technologies. In fact, there are a large number of companies that have reserved patents.